Carcinoid tumors are rare and cause either no or few nonspecific symptoms. Therefore, patients with carcinoid tumors most often present late in the course of their illness when there is already progression to an incurable state as a result of metastatic disease. At present there are neither practical population screening tests nor effective therapies and hence the 5 year survival rate is low. Due to the rareness of sporadic carcinoid tumors, large scale genetic analysis and development of sensitive and specific diagnostic tests have not been successful. While kindreds with familial carcinoid tumors that are not ascribable to known genetic syndromes are exceedingly rare, they provide a unique opportunity to facilitate the identification of the responsible gene mutation. In addition, the mutated gene in the rare familial form may also underlie the origin of the more common sporadic occurrence of carcinoid tumors. We propose to study families in which there are at least two known affected members with carcinoid tumors. We aim to diagnose patients with early and therefore potentially curable occult disease. Therefore, family members who have up to a 50% lifetime risk of harboring a carcinoid tumor will undergo an intensive diagnostic evaluation using biochemical, endoscopic and imaging modalities at initial and subsequent two year follow up encounters. Early phenotypic assignment of affected family members and collection of germline and tumoral DNA from multiple kindreds should also facilitate the genetic analysis leading to the identity of the disease gene. Evaluation of affected family members at varying stages of disease will contribute to our understanding of the natural history of carcinoid tumors and the relative utility of a variety of diagnostic and surveillance tests. Hopefully, such knowledge gained will also be applicable to patients with carcinoid tumors occurring sporadically or in the setting of other familial cancer syndromes. Thus far, we have found that familial and sporadic carcinoids are clinically indistinguishable except for the multiple synchronous primary tumors observed in most familial cases. Nearly 34% of asymptomatic relatives older than age 50 were found to have occult tumors; these tumors could be cleared surgically from 87% of these individuals (20 of 23). In one large family, linkage analysis and whole-exome sequencing identified a germline 4-bp deletion in the gene inositol polyphosphate multikinase (IPMK), which truncates the protein. This mutation was detected in all 11 individuals with small intestinal carcinoids and in 17 of 35 family members whose carcinoid status was unknown. Mutant IPMK had reduced kinase activity and nuclear localization, compared with the full-length protein. This reduced activation of p53 and increased cell survival. In summary, we found that small intestinal carcinoids can occur as an inherited autosomal dominant disease. The familial form is characterized by multiple synchronous primary tumors, which might account for 22%-35% of cases previously considered sporadic. Relatives of patients with familial carcinoids should be screened to detect curable early stage disease. IPMK haploinsufficiency promotes carcinoid tumorigenesis. We continue to enroll and screen new families and apply linkage analysis and WGS to identify other susceptibility genes, follow surgically treated patients for recurrent disease and screen carriers for emergence of occult tumor. As a result, we now follow nearly 100 families and have evaluated 396 inpatients, including 115 previously diagnosed and 298 asymptomatic relatives of which 64 (20%) screened positive (87% stage I-IIIB, 96% R0). Patient derived samples and mouse studies suggest that pathological stem cell dynamics in the small intestine of the +4 reserve ISC and altered stem cell dynamics of the intestinal epithelium may contribute to the development of EC derived small intestinal tumors.